Powdery mildews are fungal diseases that affect a wide range of plants including cereals, grasses, vegetables, ornamentals, weeds, shrubs, fruit trees, broad-leaved shade and forest trees, that is caused by different species of fungi in the order Erysiphales. The disease is characterized by spots or patches of white to grayish, talcum-powder-like growth that produce tiny, pinhead-sized, spherical fruiting structures (the cleistothecia or overwintering bodies of the fungus), that are first white, later yellow-brown and finally black. The fungi that cause powdery mildews are host specific and cannot survive without the proper host plant. They produce mycelium (fungal threads) that grow only on the surface of the plant and feed by sending haustoria, or root-like structures, into the epidermal cells of the plant. The fungi overwinter on plant debris as cleistothecia or mycelia. In the spring, the cleistothecia produce spores that are moved to susceptible hosts by rain, wind or insects.
Powdery mildew disease is particularly prevalent in temperate and humid climates, where they frequently cause significant yield losses and quality reductions in various agricultural settings including greenhouse and field farming. This affects key cereals (e.g. barley and wheat), horticultural crops (e.g. grapevine, pea and tomato) and economically important ornamentals (e.g. roses). Limited access to natural sources of resistance to powdery mildews, rapid changes in pathogen virulence and the time consuming introgression of suitable resistance genes into elite varieties has led to the widespread use of fungicides to control the disease. This has, not surprisingly, led to the evolution and spread of fungicide resistance, which is especially dramatic amongst the most economically important powdery mildews.
Downy mildew diseases are caused by oomycete microbes from the family Peronosporaceae that are parasites of plants. Peronosporaceae are obligate biotrophic plant pathogens and parasitize their host plants as an intercellular mycelium using haustoria to penetrate the host cells. The downy mildews reproduce asexually by forming sporangia on distinctive white sporangiophores usually formed on the lower surface of infected leaves. These constitute the “downy mildew” and the initial symptoms appear on leaves as light green to yellow spots. The sporangia are wind-dispersed to the surface of other leaves. Depending on the genus, the sporangia may germinate by forming zoospores or by germ-tube. In the latter case, the sporangia behave like fungal conidia and are often referred to as such. Sexual reproduction is via oospores.
Most Peronosporaceae are pathogens of herbaceous dicots. Some downy mildew genera have relatively restricted host ranges, e.g. Basidiophora, Paraperonospora, Protobremia and Bremia on Asteraceae; Perofascia and Hyaloperonospora almost exclusively on Brassicaceae; Viennotia, Graminivora, Poakatesthia, Sclerospora and Peronosclerospora on Poaceae, Plasmoverna on Ranunculaceae. However, the largest genera, Peronospora and Plasmopara, have very wide host ranges.
In commercial agriculture, downy mildews are a particular problem for growers of crucifers, grapes and vegetables that grow on vines. Peronosporaceae of economic importance include Plasmopara viticola which infect grapevines, Peronospora tabacina which causes blue mold on tobacco, Bremia lactucae, a parasite on lettuce, and Plasmopara halstedii on sunflower.
Rusts (Pucciniales, formerly Uredinales) are obligate biotrophic parasites of vascular plants. Rusts affect a variety of plants; leaves, stems, fruits and seeds and is most commonly seen as coloured powder, composed of tiny aeciospores which land on vegetation producing pustules, or uredia, that form on the lower surfaces. During late spring or early summer, yellow orange or brown, hairlike or ligulate structures called telia grow on the leaves or emerge from bark of woody hosts. These telia produce teliospores which will germinate into aerial basidiospores, spreading and causing further infection.
In the monocot barley (Hordeum vulgare, Piffanelli et al. Nature. 2004 430 (7002):887-91) and the dicots Arabidopsis thaliana (Consonni et al. Nat Genet. 2006 38(6):716-20), tomato (Solanum lycopersicum, Bai et al. Mol Plant Microbe Interact. 2008 21(1):30-9) and pea (Pisum sativum, Humphry et al. Mol Plant Pathol. 2011 Apr. 21. EPUB), loss-of-function mutations in MLO (Mildew Resistance Locus O) genes confer highly effective broad-spectrum powdery mildew resistance. MLO resistance appears to act early and typically terminates fungal pathogenesis before invasion of the first host cell. The exceptional efficacy and longevity of MLO resistance, has resulted in elite barley lines carrying introgressed MLO alleles being successfully used in European agriculture for about three decades. However, MLO mutants have several undesirable agronomic qualities including environment-dependent necrotic leaf spotting and reduced yields (Molina-Cano et al. Theor Appl Genet. 2003 107(7):1278-87). In addition, barley MLO mutants also show enhanced susceptibility to the hemibiotroph Magnaporthe grisea and the necrotroph Bipolaris sorokinianaare. Lab studies with Arabidopsis powdery resistant MLO mutants suggest that these agronomic defects, including others such as spontaneous cell wall appositions, cell death, senescence-like chlorosis and enhanced susceptibility to Alternaria alternata, A. brassicicola and Phytophthora infestans (necrotrophic Alternaria spp. and hemibiotrophic P. infestans), respectively, are pleiotropic effects not simply linkage drag (Consonni et al. Nat Genet. 2006 38(6):716-20).
Recently a method to increase resistance to soybean rust via the transgenic knockdown of MLO genes has been disclosed (Markus et al. United States Patent Application 20100192254).